1. Field of the Invention
The present invention relates generally to high-intensity, metal halide arc discharge lamps having fused silica arc tubes filled with a mixture including sodium halides and at least one additional metal halide, and optionally mercury, and, more particularly, to a metal silicate coating present on the inner surface of the arc tube for extending the useful life of the lamp by reducing loss of the metallic portion of the fill by, for example, sodium ion diffusion through the fused silica arc tube or reaction of the metal halides with the fused silica arc tube, and a corresponding undesirable buildup of free halogen in the arc tube.
2. Background of the Art
Metal halide arc discharge lamps having a construction typical of this type of lamp are shown, for example, in U.S. Pat. Nos. 4,047,067 and 4,918,352 (electroded), and 5,032,762 (electrodeless), the disclosures of which are incorporated herein by reference. Metal halide lamps of this type generally contain a filling of light emitting metals including sodium, and rare earth elements in the form of halides, commonly the iodide, and optionally mercury, in arc tubes composed of, for example, fused silica, alumina, and crystalline synthetic sapphire.
The lifetime of such lamps is frequently limited, however, by the loss of the metallic portion of the metal halide fill during lamp operation due to sodium ion diffusion and/or reaction of the metal halides with the fused silica arc tube and the corresponding building of free halogen in the arc tube. The term "free halogen" as used herein refers to volatile forms of halogens and halides present at lamp operating temperatures and includes free halogen released as sodium ions diffuse through the arc tube wall, as well as free halides, i.e., halide reaction products of halogen with any of the various constituents of the fill.
The mobility of the sodium ion is such that the arc tubes are relatively porous to it and, during lamp operation, sodium will pass from the hot arc plasma through the arc tube wall to the cooler region between the arc tube and the outer jacket or envelope of the lamp and condense on the outer envelope and parts. The lost sodium is thus unavailable to the discharge and can no longer contribute its characteristic emission so that the light output gradually diminishes and the color shifts from white toward blue. The arc becomes constricted and, in a horizontally operating lamp particularly, may bow against the arc tube wall and soften it. Also, loss of sodium causes the operating voltage of the lamp to increase and it may rise to the point where the arc can no longer be sustained ending the life of the lamp.
An additional source of loss of the metallic portion of the fill during lamp operation and corresponding buildup of free halogen is the chemical reaction of metal halides in the fill with the silicon dioxide, SiO.sub.2, of the inner surface of the fused silica arc tube which produces, for example, metal silicate crystals and silicon tetraiodide. This results in a color shift in the lamp, wall darkening, and lumen loss.
Thus, the industry has been searching for ways to prevent or minimize sodium loss by diffusion through the fused silica arc tubes of metal halide arc discharge lamps, as well as to reduce or prevent reactions of the ionizable, light-emitting metal species in the fill with the fused silica walls of the arc tubes. Attempts to solve these problems have included providing aluminum silicate and titanium silicate layers on the outside surface of the arc tube (U.S. Pat. Nos. 4,047,067 and 4,017,163, respectively). U.S. Reissue Pat. No. 30,165 discloses vitreous metal phosphates and arsenates as coatings for the inner surfaces of ceramic and silica arc tubes. U.S. Pat. No. 3,984,590 discloses aluminum phosphates and U.S. Pat. No. 5,032,762 discloses beryllium oxide as coatings for the inner surfaces of arc tubes, the disclosures of all of the foregoing being incorporated herein by reference.
Despite the coating advances of the prior art, however, the problems of loss of the light-emitting, metallic portion of the fill by diffusion or reaction and the corresponding buildup of free halogen in the arc tube have not been heretofore satisfactorily solved.
Accordingly, it is an object of the present invention to provide means for reducing loss by diffusion and/or reaction of the metallic portion of the fill of an arc tube of a metal halide arc discharge lamp and hence reducing corresponding buildup of free halogen thereby extending the useful life of the lamp.
It is another object of the present invention to provide an improved arc tube and an improved metal halide discharge lamp including the improved arc tube having the aforesaid means.